The action of aldosterone on target tissues is via the induction of RNA and protein synthesis. The first step in this process is binding of steroid to a cytoplasmic receptor. The concept of an allosteric receptor, one which exists in two states, was suggested on the basis of work with aldosterone and an aldosterone-antagonist, spirolactone. In this model, agonists bound to the receptor have high affinity for nuclear target sites while antagonists bound to the same receptor have low affinity for these sites, and therefore antagonize the action of aldosterone. Very little is known about the localization of aldosterone target cells within the kidney and the nature of the interaction between receptor and chromatin within these cells. This study attempts to identify the target cells, isolate them and study the interaction of receptor with chromatin. The importance of these studies lies in a need to know how steroid hormones affect gene regulation in normal and neoplastic tissues.